Kean returns to Turin two years after the forward left for the Premier League in a £27million (€31.5m) move, having signed a five-year deal with the Toffees.
Juve will pay €7m (£6m) for the striker's loan spell, payable over two years. They will be obliged to re-sign him on a permanent basis for a fee that could reach €31m (£26.6m) if certain objectives are met.
The Italy international's arrival follows Ronaldo's switch back to Manchester United, with Massimiliano Allegri eager to bolster his attacking options.
Kean impressed on loan at Paris Saint-Germain last season, scoring 13 times in 26 Ligue 1 outings.
But he failed to make an impact at Goodison Park, with his 32 top-flight outings yielding only two goals.
The appointment of Rafael Benitez at Everton might have ushered in a fresh start for Kean, but that will not now materialise.
Kean was sent off in Tuesday's EFL Cup win at Huddersfield Town, while he featured only very briefly in the 2-2 draw at Leeds United in the Premier League.
Kean made his Juve debut aged just 16 years, eight months and 21 days in November 2016, becoming the club's youngest ever player.
He made 16 Serie A appearances in in his first spell at Juve, scoring seven goals, while he won the Scudetto twice and the Coppa Italia once.